Yes you can walk from Holiday Inn to DTD. Its about a 10-12 minute walk. The bus from Holiday Inn to MK will drop you off at the TTC.
The only buses that go to the main gate of MK are the Disney resort buses.
Wanted to ask are you staying at the Holiday Inn on Hotel Plaza Blvd or the Holiday Inn - Main Gate? Because a lot people assume that the Holiday Inn -Main Gate is close to Disneys main gate and its not! The Holiday Inn on Hotel Plaza Blvd is the one that is within walking distance to DTD.